The invention relates to a process for producing polyamines of the diphenyl methane series and to a process for producing polyisocyanates of the diphenyl methane series having a reduced color value obtained by reacting the corresponding polyamine with phosgene.
Polyisocyanates of the diphenyl methane series are isocyanates and mixtures of isocyanates of the following type: 
Corresponding polyamines of the diphenyl methane series are compounds and mixtures of compounds of the following type: 
Large scale production of isocyanates by reacting amines with phosgene in solvents is known and described in detail in the literature (see, e.g., Ullmanns Enzyklopxc3xa4die der technischen Chemie, 4th Edition, Vol. 13, pages 347 to 357, Verlag Chemie GmbH, Weinheim, 1977). The polyisocyanate mixture used as the polyisocyanate component in the production of polyurethane foams and other polyurethane plastics obtained by the polyaddition process, is produced on the basis of this process.
It is generally known that undesirable dyes or coloring components are also formed in this process, and are retained even during further processing to produce polyurethane foams or other polyurethane plastics. Although the inherent color of the polyisocyanate polyaddition products does not adversely affect the mechanical properties thereof, substantially colorless products are desired by the consumer. The extinction at various wavelengths serves as a measure for the coloration of the polyisocyanate.
Therefore, the reduction in the color value of polyisocyanates of the diphenyl methane series has for a long time been the aim of numerous investigations and studies which are described in the literature. Thus, DE-A1-4208359, for example, describes the treatment of such isocyanates with hydrogen in the presence of carrier catalysts. DE-A1-4232769 describes the addition of amines, ureas and antioxidants to the isocyanate. DE-A1-19815055 describes an improvement in the color of polyisocyanates of the diphenyl methane series obtained by irradiation with light over a prolonged period of time. DE-A1-19804915 describes the color reduction of polyisocyanates of the diphenyl methane series by a complicated time- and temperature-graded addition of formaldehyde at the polyamine stage which is then converted into the desired isocyanate by phosgenation.
The drawback of all these procedures is that they are technically complex and/or necessitate the use of auxiliaries or are less efficient.
The object of the present invention is therefore to provide a technically simple and reliable process for producing polyisocyanates of the diphenyl methane series having low color values. A further object of the present invention is to provide a simple process for producing polyamines of the diphenyl methane series from which polyisocyanates of the diphenyl methane series with low color values can be produced by phosgenation.
These and other objectives which will be apparent to those skilled in the art are accomplished by neutralizing a polyamine-containing reaction mixture with a base, separating the aqueous and organic phases of the neutralized mixture and adding base to the separated organic phase. The separated organic phase to which base has been added may then be phosgenated to produce the corresponding polyisocyanate.